I appreciate the detailed correction. I will reflect the corrections in the next patch. plus, the explanation in the code will be fixed.
> -----Original Message----- > From: Petr Mladek [mailto:pmla...@suse.com] > Sent: Wednesday, May 30, 2018 5:32 PM > To: Sergey Senozhatsky <sergey.senozhatsky.w...@gmail.com> > Cc: Hoeun Ryu <hoeun....@lge.com.com>; Sergey Senozhatsky > <sergey.senozhat...@gmail.com>; Steven Rostedt <rost...@goodmis.org>; > Hoeun Ryu <hoeun....@lge.com>; linux-kernel@vger.kernel.org > Subject: Re: [PATCH] printk: make printk_safe_flush safe in NMI context by > skipping flushing > > On Tue 2018-05-29 21:13:15, Sergey Senozhatsky wrote: > > On (05/29/18 11:51), Hoeun Ryu wrote: > > > Make printk_safe_flush() safe in NMI context. > > > nmi_trigger_cpumask_backtrace() can be called in NMI context. For > example the > > > function is called in watchdog_overflow_callback() if the flag of > hardlockup > > > backtrace (sysctl_hardlockup_all_cpu_backtrace) is true and > > > watchdog_overflow_callback() function is called in NMI context on some > > > architectures. > > > Calling printk_safe_flush() in nmi_trigger_cpumask_backtrace() > eventually tries > > > to lock logbuf_lock in vprintk_emit() but the logbuf_lock can be > already locked in > > > preempted contexts (task or irq in this case) or by other CPUs and it > may cause > > The sentence "logbuf_lock can be already locked in preempted contexts" > does not > make much sense. It is a spin lock. It means that both interrupts and > preemption are disabled. > > I would change it to something like: > > "Calling printk_safe_flush() in nmi_trigger_cpumask_backtrace() eventually > tries > to lock logbuf_lock in vprintk_emit() that might be already be part > of a soft- or hard-lockup on another CPU." > > > > > deadlocks. > > > By making printk_safe_flush() safe in NMI context, the backtrace > triggering CPU > > > just skips flushing if the lock is not avaiable in NMI context. The > messages in > > > per-cpu nmi buffer of the backtrace triggering CPU can be lost if the > CPU is in > > > hard lockup (because irq is disabled here) but if panic() is not > called. The > > > flushing can be delayed by the next irq work in normal cases. > > I somehow miss there a motivation why the current state is better than > the previous. It looks like we exchange the risk of a deadlock with > a risk of loosing the messages. > > I see it the following way: > > "This patch prevents a deadlock in printk_safe_flush() in NMI > context. It makes sure that we continue and eventually call > printk_safe_flush_on_panic() from panic() that has better > chances to succeed. > > There is a risk that logbuf_lock was not part of a soft- or > dead-lockup and we might just loose the messages. But then there is a high > chance that irq_work will get called and the messages will get flushed > the normal way." > > > > Any chance we can add more info to the commit message? E.g. backtraces > > which would describe "how" is this possible (like the one I posted in > > another email). Just to make it more clear. > > I agree that a backtrace would be helpful. But it is not a must to > have from my point of view. > > The patch itself looks good to me. > > Best Regards, > Petr
